The overall objective of this research is to test the hypothesis that specific morphological changes in the neurons controlling gonadotropin hormone release (GnRH neurons) are part of the mechanism initiating puberty. The novel combination of neuroanatomical techniques with assessment of the physiological output of the GnRH system promises to make a significant contribution to the study of puberty. The male Djungarian hamster, as the animal model, provides an unique opportunity for this proposal because sexual maturation is profoundly influenced by photoperiod; changes in day length can delay or initiate puberty. The techniques of quantitative immunocytochemistry will be used to delineate important features of the GnRH system including: the size, location, number and distribution of GnRH neurons, throughout development. Since six distinct types of GnRH neurons have been identified it is proposed that the onset of puberty is a consequence of specific changes in the morphology of a class of GnRH neurons or changes in population characteristics within the GnRH neural system. Experiments will be conducted to determine if delayed puberty blocks this developmental change in the GnRH system. The role of photoperiod and gonadal steroids in the feedback regulation of gonadotropin hormones and upon GnRH neuron morphology will be explored. Finally, the contribution of factors within and outside of the hypothalamus, for the process controlling the onset of puberty will be studied. This research approach will yield important new information to extend the current working hypothesis for puberty from the systemic to the cellular level. The results will have broad application to the understanding of normal and delayed puberty.